DESCRIPTION: Elevated apoB levels are associated with an increased risk of coronary heart disease. Hypobetalipoproteinemia (HBLP) is characterized by apoB levels less than the 5 percentile. The applicant has sequenced mutations for truncated forms of apoB-67, apoB-55 and apoB-44.4 which causes HBLP, described a kindred from the Framingham Heart Study with HBLP due to an unidentified apoB gene mutation and purified apoB-67 containing lipoprotein particles. Heterozygous apoB-67 subjects have one normal allele making apoB-100; therefore, apoB levels would be predicted to be at least 50 percent of normal; however, they are 24 percent of normal. The applicant has shown that these lower than expected levels result from decreased production of VLDL apoB-100, LDL apoB-100 and apoB-67, increased catabolism of VLDL apoB-100, and increased direct removal of apoB-67 from VLDL. The applicant proposes to study mechanisms for these observations. Specific aim 1 is to locate the apoB gene mutation in the Framingham kindred. Specific aim 2 is to perform stable isotope studies in the apoB-55 and apoB-44.4 kindreds to determine if apoB metabolism for these shorter truncations is similar to that for apoB-67. In specific aim 3, apoB-100 synthesis will be studied in heterozygous apoB-70 transgenic mice. If it is 25-25 percent of normal litter mates, the mechanism for this reduction in apoB-100 levels will be studied in hepatocytes isolated from the transgenic mice. In specific aim 4, size and composition of VLDL will be compared in apoB-67 subjects and controls to determine if larger size or compositional changes account for the faster catabolism of VLDL apoB-100.